BAY 11-7082 antagonizes I-κB kinase-β preventing nuclear translocation of nuclear factor-κB (NF-κB); it also inhibits NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome activation. NF-κB is involved in psoriasis, whereas the role of NLRP3 is controversial. We investigated BAY 11-7082 effects in an experimental model of psoriasis-like dermatitis. Psoriasis-like lesions were induced by a topical application of imiquimod (IMQ) cream (62.5 mg/day) on the shaved back skin of C57BL/6 and NLRP3 knockout (KO) mice for 7 consecutive days. Sham psoriasis animals were challenged with Vaseline cream. Sham and IMQ animals were randomized to receive BAY 11-7082 (20 mg/kg/i.p.) or its vehicle (100 μl/i.p of 0.9% NaCl). Skin of IMQ animals developed erythema, scales, thickening and epidermal acanthosis. IMQ skin samples showed increased expression of pNF-κB and NLRP3 activation. BAY 11-7082 blunted epidermal thickness, acanthosis and inflammatory infiltrate. BAY 11-7082 reduced pNF-κB, NLRP3, tumour necrosis factor-α (TNF-α), interleukin (IL)-6 and IL-1β expression, blunted the phosphorylation of signal transducer and activators of transcription 3 (STAT3) and decreased IL-23 levels. In addition, BAY 11-7082 reawakened the apoptotic machinery. NLRP3 KO animals showed a reduced total histological score but persistent mild acanthosis, dermal thickness and expression of pNF-κB and pSTAT3, following IMQ application. Our data suggest that BAY 11-7082 might represent an interesting approach for the management of psoriasis-like dermatitis depending on the dual inhibition of NF-κB and NLRP3.

Clinical perspectives

  • Psoriasis is one of the main immune-mediated inflammatory skin disorder affecting 1–3% of the population worldwide. The transcription factors NF-κB and STAT3 are involved in psoriasis, while the role of NLRP3 inflammasome is controversial. BAY 11-7082 antagonizes I-κB kinase-β preventing nuclear translocation of NF-κB and also inhibits NLRP3 inflammasome activation. We investigated BAY 11-7082 effects in an experimental model of psoriasis-like dermatitis.

  • The results of the present study show that BAY 11-7082 is able to reduce inflammation, inhibiting both NF-κB and NLRP3, and to improve apoptosis.

  • These findings may be easily translated into clinical practice to generate meaningful health outcomes. BAY 11-7082 beneficial effect is dependent on a NF-?B and NLRP3 inhibition that interrupts the pathological mechanisms underlying the triggering and the maintenance of psoriasis lesion.

Introduction

Psoriasis is one of the main immune-mediated inflammatory skin disorders affecting 0.51–11.43% of the population worldwide [1]. The hallmark of psoriasis is the appearance of erythematous scaly plaques and psoriatic skin is characterized by acanthosis, erythema and inflammatory infiltrate [2]. Pathogenesis of the disease is still poorly understood but several studies have demonstrated that the interleukin (IL)-23/T helper (Th)17 axis and the inflammatory cytokines produced by Th17 are involved both in pathogenesis and in the development of psoriatic disorder [37].

The increased production of pro-inflammatory cytokines and chemokines is regulated by one of the main transcriptional factors: nuclear factor-κB (NF-κB) [8]. NF-κB plays a crucial role in inflammatory processes involved in psoriasis, activating molecular patterns and promoting histological hallmarks such as the conversion of pre-psoriatic lesions to psoriatic ones [9]. Previous clinical studies have already demonstrated that psoriatic skin is characterized by an increased expression of pNF-κB [1012]. Under physiological conditions, NF-κB is maintained in an inactive state in the cytoplasm of cells bound to its inhibitor called I-κB; when I-κB is phosphorylated by I-κB kinase, NF-κB rapidly enters the nucleus and activates gene expression [13]. NF-κB regulates not only the transcription of genes involved in inflammation but also of those implicated in the apoptotic process [14]. NF-κB may inhibit p53-induced transcription of the apoptotic BAX gene in several types of cancer cell lines, blunting programmed cell death and stimulating proliferation; by contrast, its inhibition may lead to an increased expression of BAX, thus stimulating the apoptosis machinery [15]. Indeed, psoriasis is characterized by abnormal apoptosis, particularly in keratinocytes [16]. Activation of NF-κB in these cell types leads to epidermal hyperproliferation related to anti-apoptotic mechanisms thus prolonging cell lifespan and survival [17,18].

Tumour necrosis factor-α (TNF-α) and IL-6 are considered, among the cytokines produced, the most important mediators of the inflammatory process in psoriasis [19,20]. TNF-α, in turn, may activate NF-κB with a positive feedback mechanism [21], amplifying the inflammatory process in skin scales. IL-6 not only worsens inflammation in psoriatic lesions [22], but also may be responsible for the activation of signal transducer and activators of transcription 3 (STAT3). STAT3 is involved in several inflammatory skin diseases and its activation seems to be related to IL-23 pathway, also implicated in psoriasis [23]. Psoriatic skin plaques show increased phosphorylation of STAT3 (pSTAT3) both in humans and in transgenic mice [24]. When STAT3 is activated, it modulates gene expression involved in proliferation [25]; furthermore, it seems that the induction of a certain gene subset requires co-operation between STAT3 and NF-κB pathways [26].

Inflammasomes are multi-protein complexes also involved in the activation of inflammatory response. Among the inflammasomes, one of the most studied is the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome. The NLRP3 inflammasome complex formation involves NLRP3, an apoptosis-associated speck-like protein containing a caspase activation recruitment domain (ASC) and procaspase-1. Procaspase-1 is a zymogen that is cleaved following inflammasome assembly into the 20 kDa (p20) and 10 kDa (p10) subunits that form the active caspase-1 p10/p20 tetramer. Subsequently, activated caspase-1 mediates pro-IL-1β processing and mature IL-1β release [27]. NLRP3 is particularly activated in skin inflammatory diseases, but its involvement in psoriasis is still questioned [2830].

Scientific evidence has demonstrated that BAY 11-7082 inhibits NLRP3 inflammasome [3134]. However, this compound also selectively targets I-κB kinase-β thus blocking NF-κB nuclear translocation. As far as we know BAY 11-7082 has not been previously investigated in experimental psoriasis. In the present study, we investigated the effects of BAY 11-7082 in an experimental animal model of psoriasis-like dermatitis and we investigated the underlying mechanism(s) of action.

Materials and methods

Induction of IMQ-induced psoriasis-like lesions and treatment

C57BL/6 mice (n=28) and NLRP3 knockout (KO) (n=21) were used in the present study and were obtained from Charles River Laboratories. During the experiment, mice were maintained in the Animal Facility of the Department of Clinical and Experimental Medicine, under controlled environmental conditions (12 h light/dark cycle, at temperature 24°C), and provided with standard food and water ad libitum.

The experiment was performed in compliance with the standards for care and use of animals as affirmed in the Guide for the Care and Use of Laboratory Animals (Institute of Laboratory Animal Resources, National Academy of Sciences, Bethesda, MD, U.S.A.) and the ARRIVE guidelines [35]; the procedures were evaluated and approved by the Ethics Committee of the University of Messina.

Psoriasis-like lesions were induced by a topical daily application of a commercially available imiquimod (IMQ) cream (62.5 mg/day for 7 days) on the shaved back skin of 6-week-old C57BL/6 mice (n=14) for 7 consecutive days. Sham psoriasis animals (n=14) (Sham IMQ) were challenged with Vaseline cream. Sham IMQ and IMQ animals were then randomized to receive either BAY 11-7082 (20 mg/kg/i.p.) or its vehicle (100 μl/i.p of 0.9% NaCl saline solution). IMQ psoriasis was also induced in (n=14) 6-week-old male NLRP3−/− KO mice on C57BL/6 background to evaluate NLRP3 inflammasome involvement in the pathogenesis of IMQ-induced psoriasis-like dermatitis. A group (n=7) of IMQ–NLRP3−/− also received BAY 11-7082 (20 mg/kg/i.p.) for the duration of the experiment. Sham psoriasis animals (n=7; Sham NLRP3−/− KO) were challenged with Vaseline cream. During the experiment mice were observed to evaluate the appearance of scales, typical of the psoriatic lesions. Four hours after the last administration (day 7) of either IMQ cream or Vaseline, mice were killed and skin samples were obtained from each animal to perform molecular, histological and immunohistochemical analysis.

To identify BAY 11-7082 dose, we carried out preliminary experiments with different doses (5, 10 and 20 mg/kg) of the compound, titrated against the histological damage. The dose of 20 mg/kg was the most effective and therefore we used this dosage in the study to keep to the minimum, for ethical issues, the number of experimental animals. This dose was similar to that used in a previously published paper investigating the effects of BAY 11-7082 on depressed wound healing in diabetic animals [31]. Since psoriasis is a systemic disease we chose the intraperitoneal route of administration.

Histologic evaluation

After removal, skin samples were immediately fixed in 10% buffered formalin at room temperature for at least 24 h. Sections were dehydrated in graded ethanol, cleared in xylene and embedded in paraffin according to routine techniques. Five-micrometre-thick sections of paraffin-embedded tissues were mounted on glass slides, hydrated in distilled water and then stained with haematoxylin and eosin. In particular, histological slides were examined at ×10 to ×20 magnification to observe skin structure, possible morphologic alterations and the effects following treatment (i.e. erythema, scales, thickening, epidermal acanthosis and inflammation). Two observers, blinded to the experimental protocol carried out assessment of tissue changes on coded samples. Dermal thickness, defined as the thickness of skin from the top of the granular layer to the junction between the dermis and s.c. fat was examined, using the Leica application suite software (Leica Microsystems), as previously described [36]. Ten random measurements were taken per section. The results were expressed in micrometres as mean values of dermal thickness for each group. Baker's scoring system was also used to evaluate the pathological alterations on a scale ranging from 0 to 10, as recently described [37].

Immunohistochemical evaluation of cytokeratin 6 and active caspase-3

Paraffin-embedded tissues were sectioned (5 μm), rehydrated, and antigen retrieval was performed by using 0.05 M sodium citrate buffer (pH 6.0) in a microwave for 5 min. Tissues were treated with 3% hydrogen peroxide to block endogenous peroxidase activity, and with normal horse serum (Vector Laboratories) to prevent non-specific staining. Primary antibodies against either cytokeratin 6 (1:10; ab18586, Abcam) or active caspase-3 (1:100; #3015-100, BioVision) were used and the slides were kept overnight at 4°C in a humid box. Slides were then washed in PBS, the appropriate secondary antibody was added and the ABC system (all from Vectastain Elite ABC Kit, Vector Laboratories) was used to detect antibody localization. The location of the reaction was visualized with diaminobenzidine tetrahydrochloride (DAB; Sigma–Aldrich). Slides were counterstained with haematoxylin, dehydrated and mounted with coverslips. All slides were coded and evaluated by a pathologist at ×5 to ×40 magnification with a Leica microscope (Leica Microsystems).

Western blot analysis

After removal, skin samples were homogenized in lysis buffer (25 mM Tris/HCl, pH 7.4, 1.0 mM EGTA, 1.0 mM EDTA, 0.5 mM PMSF, 10 μg/ml aprotinin, 10 μg/ml leupeptin, 10 μg/ml pepstatin A and 10 μl/ml NP40). The homogenate was subjected to centrifugation at 15000 g for 15 min at 4°C. The concentration of total proteins was determined by using the Bio-Rad protein-assay kit. The supernatant was collected, mixed with Laemmli sample buffer (62 mmol/l Tris, pH 6.8, 10% glycerol, 2% SDS, 5% β-mercaptoethanol and 0.003% bromophenol blue) and stored at −20°C until analysis. Protein samples (10, 20 and 40 μg) were separated by electrophoresis on an SDS polyacrylamide gel (10% or 12%). Once separated, proteins were transferred on to a PVDF membrane using the transfer buffer (39 mmol/l glycine, 48 mmol/l Tris, pH 8.3 and 20% methanol) at 100 V for 1 h. Membranes were then blocked with LI-COR Blocking Buffer for 1 h at room temperature, followed by overnight incubation with primary antibodies for pNF-κB (1:1000; ab86299), total NF-κB (1:1000; ab16502), NLRP3 (1:500; ab4207), pSTAT3 (1:500; ab30647), total STAT3 (1:100; ab50761) (Abcam), BCL2 (1:500; #2876), IL-1β (1:500; #12242), caspase-3 (1:500; #9662), pJNK (1:500; #9255), total JNK (1:500 #9252), β-actin (1:500 #4967) (Cell Signaling) and cleaved caspase-3 (1:500; #3015-100; BioVision), in TBS-0.1% Tween overnight at 4°C. Following incubation, antibody was removed, membranes were washed three times for 10 min each in TBS-0.1% Tween. Protein detection was carried out using a secondary infrared fluorescent dye conjugated antibody absorbing at 800 or 700 nm. The blots were visualized using an Odyssey Infrared Imaging Scanner (Li-COR Science Tec) and quantified by densitometric analysis performed after normalization with β-actin. Results were expressed as arbitrary units (AU).

Samples were coded and the experiments were repeated three times. Results were expressed as integrated intensity compared with those of control animals measured within the same batch.

Determination of IL-1β in skin lysates

The amount of IL-1β in skin of treated animals was determined at day 7, by commercially available mouse-specific ELISA assay kit (ab100-705; Abcam), following manufacturer's instructions. IL-1β detection range was 2.74–2000 pg/ml.

Real-time PCR for IL-6, TNF-α and IL-23

Gene expression study was performed following extraction of total mRNA, isolated from skin tissue using TRIZOL reagent (Invitrogen), following the manufacturer's protocol. The first strand of cDNA was synthesized from 5 μg of total RNA using the High-Capacity cDNA Archive Kit (Life Technologies). Gene expression of IL-6 (Mm00446190_m1), TNF-α (Mm00443258_m1) and IL-23 (Mm00518984_m1) was quantified by using TaqMan inventoried probes (Life Technologies) through the SDS 7300 Real Time PCR instruments (Applied Biosystems). β-Actin (catalogue number: 4352341E; Life Technologies) was used as endogenous control and data were analysed using the 2−ΔΔct method. We assumed the 2−ΔΔct from Sham samples as calibrator for data calculation.

Statistical analysis

All data are expressed as means ± S.D. Comparisons between different treatments were analysed by one-way or two-way ANOVA for non-parametric variables with Tukey's post-test for intergroup comparisons. Mann–Whitney U-test was used to analyse the histological scores. The possibility of error was set at P<0.05 and it was considered statistically significant. All analyses were performed using Stata/IC 12.0 (StataCorp LP). Graphs were drawn using GraphPad Prism (version 5.0 for Windows).

Results

IMQ-induced skin lesions

IMQ and Vaseline were applied on the shaved skin of mice for 7 consecutive days. Sham mice (n=14) treated with either Vaseline (n=7) or BAY 11-7082 (n=7) did not show signs of inflammation, and skin had a normal architecture (Figures 1A–1D). IMQ animals (n=7) showed signs of erythema and scales starting from day 3 of application, resembling psoriatic lesions in humans (Figure 1E). Microscopically the epidermal changes were represented by thickening due to the increase in the epidermal layers, acanthosis and papillomatosis (“psoriasiform hyperplasia”) involving also the suprapapillary plates and mild orthokeratosis; the derma showed an inflammatory infiltrate with granulocytes and lymphocytes and dilated capillary vessels at the tips of the papillae with some extravasated erythrocytes (Figure 1F). Administration of BAY 11-7082 produced a reduction in epidermal thickness (Figure 1G) with loss of papillomatosis and residual mild acanthosis; the inflammatory infiltrate was reduced with few lymphocytes in the dermis (Figure 1H).

Histological changes

Figure 1
Histological changes

Representative macroscopic appearance of Sham WT (A), Sham WT + BAY 11-7082 (C), IMQ WT (E), IMQ WT + BAY 11-7082 (G), Sham NLRP3−/− KO (I), IMQ NLRP3−/− KO (M); IMQ NLRP3−/− KO + BAY 11-7082 (O) mice back skin at day 7; scale bar 50 μm. Skin of WT animals showed erythema and scales (E) following IMQ topical application; skin of WT and NLRP3−/− KO mice who received Vaseline application showed no change (A and I). Skin of WT mice treated with BAY 11-7082 and skin of NLRP3−/− KO mice, both receiving IMQ, did not show scales or erythema (G and M). H&E stained sections of skin at day 7 examined under light microscopy (original magnification ×10) of Sham WT (B), Sham WT + BAY 11-7082 (D), IMQ WT (F), IMQ WT + BAY 11-7082 (H), Sham NLRP3−/− KO (L) and IMQ NLRP3−/− KO (N). Skin of WT and KO mice treated with Vaseline had a normal architecture (B and L). IMQ WT mice showed an increase in epidermal layers, acanthosis, papillomatosis and mild orthokeratosis with dermal inflammatory infiltrate (F). The administration of BAY 11-7082 determined the reduction in epidermal thickness with loss of papillomatosis and residual mild acanthosis; the inflammatory infiltrate was also reduced (H). Skin of IMQ NLRP3−/− KO mice showed an almost normal epidermal stratification and mild acanthosis (N) compared with IMQ WT mice (F). IMQ NLRP3−/− KO mice treated with BAY 11-7082 showed an almost normal thickness without signs of acanthosis (P).

Figure 1
Histological changes

Representative macroscopic appearance of Sham WT (A), Sham WT + BAY 11-7082 (C), IMQ WT (E), IMQ WT + BAY 11-7082 (G), Sham NLRP3−/− KO (I), IMQ NLRP3−/− KO (M); IMQ NLRP3−/− KO + BAY 11-7082 (O) mice back skin at day 7; scale bar 50 μm. Skin of WT animals showed erythema and scales (E) following IMQ topical application; skin of WT and NLRP3−/− KO mice who received Vaseline application showed no change (A and I). Skin of WT mice treated with BAY 11-7082 and skin of NLRP3−/− KO mice, both receiving IMQ, did not show scales or erythema (G and M). H&E stained sections of skin at day 7 examined under light microscopy (original magnification ×10) of Sham WT (B), Sham WT + BAY 11-7082 (D), IMQ WT (F), IMQ WT + BAY 11-7082 (H), Sham NLRP3−/− KO (L) and IMQ NLRP3−/− KO (N). Skin of WT and KO mice treated with Vaseline had a normal architecture (B and L). IMQ WT mice showed an increase in epidermal layers, acanthosis, papillomatosis and mild orthokeratosis with dermal inflammatory infiltrate (F). The administration of BAY 11-7082 determined the reduction in epidermal thickness with loss of papillomatosis and residual mild acanthosis; the inflammatory infiltrate was also reduced (H). Skin of IMQ NLRP3−/− KO mice showed an almost normal epidermal stratification and mild acanthosis (N) compared with IMQ WT mice (F). IMQ NLRP3−/− KO mice treated with BAY 11-7082 showed an almost normal thickness without signs of acanthosis (P).

We reproduced the experimental paradigm in NLRP3 KO mice. NLRP3−/− KO animals (n=7) who received Vaseline showed a normal skin with a normal architecture (Figures 1I and 1L). Interestingly, skin of IMQ-treated NLRP3-deficient mice (n=7) clinically presented reduced scaly skin lesions (Figure 1M) and an almost normal epidermal stratification with only very mild acanthosis (Figure 1N) compared with those of IMQ WT mice. NLRP3−/− KO animals (n=7) challenged with IMQ and treated with BAY 11-7082 showed markedly reduced psoriatic hallmarks as evidenced in Figures 1(O) and 1(P).

The above reported morphological changes affecting the structure and the organization of the skin produced differences in dermal thickness as graphically reported (Figure 2A). Moreover, the Baker's score for psoriatic lesions further confirmed that NLRP3−/− KO challenged with IMQ had significantly lower occurrence of psoriatic hallmarks (Figure 2B) as compared with wild-type animals challenged with IMQ. However, acanthosis and dermal thickness following IMQ were still persistent in NLRP3−/− KO animals, but both were abrogated by BAY 11-7082 administration (Figure 2B).

Histological scores

Figure 2
Histological scores

The histogram (A) shows the epidermal thickness measured from seven different animals; ***P<0.0001 compared with Sham and Sham WT + BAY 11-7082; *P<0.01 compared with IMQ; #P<0.05 compared with Sham NLRP3−/− KO; **P<0.05 compared with IMQ NLRP3−/− KO; §P<0.05 compared with IMQ. (B) Shows the total histology score (cumulative and for single animal) measured according to Baker's scoring system in seven different animals; ***P<0.0001 compared with Sham and Sham WT + BAY 11-7082; °°°P<0.0001 compared with IMQ; #P<0.0001 compared with Sham NLRP3−/− KO; ∧∧∧p<0.0005 compared with IMQ NLRP3−/− KO; §P<0.01 compared with IMQ.

Figure 2
Histological scores

The histogram (A) shows the epidermal thickness measured from seven different animals; ***P<0.0001 compared with Sham and Sham WT + BAY 11-7082; *P<0.01 compared with IMQ; #P<0.05 compared with Sham NLRP3−/− KO; **P<0.05 compared with IMQ NLRP3−/− KO; §P<0.05 compared with IMQ. (B) Shows the total histology score (cumulative and for single animal) measured according to Baker's scoring system in seven different animals; ***P<0.0001 compared with Sham and Sham WT + BAY 11-7082; °°°P<0.0001 compared with IMQ; #P<0.0001 compared with Sham NLRP3−/− KO; ∧∧∧p<0.0005 compared with IMQ NLRP3−/− KO; §P<0.01 compared with IMQ.

To better characterize the effect of BAY 11-7082 and of the NLRP3−/− genetic background on epidermal hyperplasia, a marker specific for hyperproliferative keratinocytes, cytokeratin 6, and the active form of the executioner caspase-3, were studied. Sham WT (Figures 3A and 3F) and Sham NLRP3−/− KO (Figures 3D and 3I) animals did not show positive staining for either cytokeratin 6 or caspase-3. Skin from the IMQ-treated animals demonstrated an increased staining for both markers in either WT (Figures 3B and 3G) and NLRP3−/− KO (Figures 3E and 3L) animals. The administration of BAY 11-7082 produced a reduced expression of cytokeratin 6 (Figure 3C) and a slight increase in active caspase-3 immunostaining (Figure 3H) in the epidermal layer.

Cytokeratin 6 and Caspase 3 immunostaining

Figure 3
Cytokeratin 6 and Caspase 3 immunostaining

Representative immunostaining for cytokeratin 6 (AE) and cleaved caspase-3 (FL) of skin sections at day 7 examined under light microscopy (original magnification ×10) of Sham WT (A and F), IMQ WT (B and G), IMQ WT + BAY 11-7082 (C and H), Sham NLRP3−/− KO (D–I) and IMQ NLRP3−/− KO (E–L). The arrows point at positive cells; scale bar 50 μm.

Figure 3
Cytokeratin 6 and Caspase 3 immunostaining

Representative immunostaining for cytokeratin 6 (AE) and cleaved caspase-3 (FL) of skin sections at day 7 examined under light microscopy (original magnification ×10) of Sham WT (A and F), IMQ WT (B and G), IMQ WT + BAY 11-7082 (C and H), Sham NLRP3−/− KO (D–I) and IMQ NLRP3−/− KO (E–L). The arrows point at positive cells; scale bar 50 μm.

BAY 11-7082 inhibits activation of pNF-κB, pSTAT3 and NLRP3

Skin of IMQ mice showed a marked activation of the phosphorylated transcription factor NF-κB. Treatment with BAY 11-7082 significantly reduced pNF-κB expression, as shown in Figure 4(A), in IMQ-treated mice. IMQ mice treated with BAY 11-7082 showed a marked reduction in pSTAT3 compared with untreated psoriatic animals that had instead a remarkable expression of this transcription factor (Figure 4B). IMQ stimulated also an increased expression of NLRP3 in WT animals and treatment with BAY-11-7082 reduced this enhanced expression (Figure 4C).

Effects of BAY 11-7082 on NLRP3 and transcription factors

Figure 4
Effects of BAY 11-7082 on NLRP3 and transcription factors

Western Blot analysis of pNF-κB (A and D), pSTAT3 (B and E) and NLRP3  (C) in skin samples of mice at day 7. Skin of WT IMQ mice showed a marked increase in the expression of pNF-κB (the histogram represents the values obtained from 10 μg of proteins), pSTAT3 (the histogram represents the values obtained from 10 μg of proteins) and NLRP3 (the histogram represents the values obtained from 20 μg of proteins) compared with Sham and Sham + BAY 11-7082 groups. Treatment with BAY 11-7082 significantly reduced the expression of these proteins. IMQ NLRP3−/− KO mice showed a significant increase in both pNF-κB and pSTAT3 expression compared with NLRP3−/− KO mice that received Vaseline. Each band has been cropped from the same film and values were obtained from seven animals per group and are expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham and Sham + BAY 11-7082; #P<0.05 compared with IMQ; *P<0.05 compared with Sham NLRP3−/− KO; #P<0.05 compared with IMQ NLRP3−/− KO.

Figure 4
Effects of BAY 11-7082 on NLRP3 and transcription factors

Western Blot analysis of pNF-κB (A and D), pSTAT3 (B and E) and NLRP3  (C) in skin samples of mice at day 7. Skin of WT IMQ mice showed a marked increase in the expression of pNF-κB (the histogram represents the values obtained from 10 μg of proteins), pSTAT3 (the histogram represents the values obtained from 10 μg of proteins) and NLRP3 (the histogram represents the values obtained from 20 μg of proteins) compared with Sham and Sham + BAY 11-7082 groups. Treatment with BAY 11-7082 significantly reduced the expression of these proteins. IMQ NLRP3−/− KO mice showed a significant increase in both pNF-κB and pSTAT3 expression compared with NLRP3−/− KO mice that received Vaseline. Each band has been cropped from the same film and values were obtained from seven animals per group and are expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham and Sham + BAY 11-7082; #P<0.05 compared with IMQ; *P<0.05 compared with Sham NLRP3−/− KO; #P<0.05 compared with IMQ NLRP3−/− KO.

Furthermore, when applied to NLRP3−/− KO animals, IMQ produced an enhanced expression of pNF-κB and pSTAT3 (Figures 4D and 4F), suggesting that on this genetic background the lack of the NLRP3 enhances the activation of other pathways involved in the psoriatic process.

Inhibition of the cytokines TNF-α, IL-6, IL-1β and IL-23 by BAY 11-7082

Skin of IMQ animals showed a marked expression of the mRNA of TNF-α, IL-6 and IL-23 compared with Sham IMQ animals. BAY 11-7082 treatment did not modify cytokine expression in Sham animals, but it markedly suppressed the mRNA expression of all the three cytokines in IMQ animals (Figures 5A–5C). IL-1β was investigated by Western blot (Figure 5D) and ELISA (Figure 5E). Skin from IMQ-challenged animals showed an augmented expression of IL-1β, which was significantly reduced by the treatment with BAY-11-7082. As expected the NLRP3−/− KO animals presented almost undetectable levels of IL-1β (Figure 5E), most of this cytokine being produced by NLRP3 activation; psoriasis induction by IMQ did not modify IL-1β production.

Effects of BAY 11-7082 on inflammatory markers

Figure 5
Effects of BAY 11-7082 on inflammatory markers

mRNA expression for IL-6 (A), TNF-α (B) and IL-23 (C) in skin samples of WT mice at day 7. IMQ mice showed a marked increase in mRNA expression for IL-6, TNF-α and IL-23 compared with Sham and Sham + BAY 11-7082 groups. Treatment with BAY 11-7082 determined a significant reduction in mRNA expression of all these cytokines. Values were obtained from seven animals per group and are expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham WT and Sham + BAY 11-7082; #P<0.05 compared with IMQ. Western Blot analysis of IL-1β (D) in skin samples of WT mice at day 7 (the histogram represents the values obtained from 20 μg of proteins). IL-1β expression increased in IMQ group; the administration of BAY 11-7082 determined a significant reduction in this expression. Each band has been cropped from the same film and values were obtained from seven animals per group and expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham and Sham + BAY 11-7082; #P<0.05 compared with IMQ. IL-1β levels (E) measured by ELISA in skin samples of Sham WT, Sham WT + BAY 11-7082, IMQ WT, IMQ WT + BAY 11-7082, Sham NLRP3−/− KO and IMQ NLRP3−/− KO obtained at day 7. Values are expressed as means and S.D. of seven animals; ***P<0.0001 compared with Sham and Sham WT + BAY 11-7082; *P<0.01 compared with IMQ. (Green fluorescent anti-rabbit secondary antibody was used for rabbit primary antibodies and red fluorescent anti-mouse secondary antibody was used for mouse primary antibodies.)

Figure 5
Effects of BAY 11-7082 on inflammatory markers

mRNA expression for IL-6 (A), TNF-α (B) and IL-23 (C) in skin samples of WT mice at day 7. IMQ mice showed a marked increase in mRNA expression for IL-6, TNF-α and IL-23 compared with Sham and Sham + BAY 11-7082 groups. Treatment with BAY 11-7082 determined a significant reduction in mRNA expression of all these cytokines. Values were obtained from seven animals per group and are expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham WT and Sham + BAY 11-7082; #P<0.05 compared with IMQ. Western Blot analysis of IL-1β (D) in skin samples of WT mice at day 7 (the histogram represents the values obtained from 20 μg of proteins). IL-1β expression increased in IMQ group; the administration of BAY 11-7082 determined a significant reduction in this expression. Each band has been cropped from the same film and values were obtained from seven animals per group and expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham and Sham + BAY 11-7082; #P<0.05 compared with IMQ. IL-1β levels (E) measured by ELISA in skin samples of Sham WT, Sham WT + BAY 11-7082, IMQ WT, IMQ WT + BAY 11-7082, Sham NLRP3−/− KO and IMQ NLRP3−/− KO obtained at day 7. Values are expressed as means and S.D. of seven animals; ***P<0.0001 compared with Sham and Sham WT + BAY 11-7082; *P<0.01 compared with IMQ. (Green fluorescent anti-rabbit secondary antibody was used for rabbit primary antibodies and red fluorescent anti-mouse secondary antibody was used for mouse primary antibodies.)

Apoptotic process is activated by BAY 11-7082 as a protective mechanism

Members of the Bcl-2 family play a crucial role in survival and their regulation either induces or inhibits apoptosis. The anti-apoptotic protein Bcl-2 was markedly expressed in IMQ animals (Figure 6A). Furthermore psoriasis animals showed an enhanced expression of pJNK and active caspase-3, (both key pro-apoptotic proteins) when compared with Sham IMQ animals (Figures 6B–6D). BAY 11-7082 treatment effectively reduced the anti-apoptotic Bcl-2 (Figure 6A) and further augmented the pro-apoptotic caspase-3 and pJNK, thus reawakening the apoptosis machinery (Figures 6B–6D).

Effects of BAY11-7082 on apoptosis markers

Figure 6
Effects of BAY11-7082 on apoptosis markers

Western Blot analysis of Bcl-2 (A), pJNK (B), caspase-3 (C) and cleaved caspase-3 (D) at day 7. The expression of the anti-apoptotic Bcl-2 (the histogram represents the values obtained from 10 μg of proteins) was increased by IMQ administration with a concomitant increase in the pro-apoptotic molecules pJNK (the histogram represents the values obtained from 40 μg of proteins) and cleaved caspase-3 (the histogram represents the values obtained from 10 μg of proteins). BAY 11-7082 reduced Bcl-2 and further increased the apoptotic markers. Each band has been cropped from the same film and values are expressed as mean and S.D. for each group. Values were obtained from seven animals per group and are expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham and Sham + BAY 11-7082; #P<0.05 compared with IMQ.

Figure 6
Effects of BAY11-7082 on apoptosis markers

Western Blot analysis of Bcl-2 (A), pJNK (B), caspase-3 (C) and cleaved caspase-3 (D) at day 7. The expression of the anti-apoptotic Bcl-2 (the histogram represents the values obtained from 10 μg of proteins) was increased by IMQ administration with a concomitant increase in the pro-apoptotic molecules pJNK (the histogram represents the values obtained from 40 μg of proteins) and cleaved caspase-3 (the histogram represents the values obtained from 10 μg of proteins). BAY 11-7082 reduced Bcl-2 and further increased the apoptotic markers. Each band has been cropped from the same film and values are expressed as mean and S.D. for each group. Values were obtained from seven animals per group and are expressed as mean and S.D. for each group. Samples were coded and the experiments were repeated three times; *P<0.05 compared with Sham and Sham + BAY 11-7082; #P<0.05 compared with IMQ.

Discussion

Skin inflammation represents one of the main hallmarks of psoriasis and a complex network of cytokines is involved in the pathogenesis of the disease [38]. Psoriatic lesions are characterized by several histological changes [39] and keratinocytes represent the key cellular type implicated in altered epidermal growth in psoriatic plaques.

Patients affected by psoriasis showed high levels of both TNF-α and IL-6 [12]; NF-κB pathway regulates the expression of pro-inflammatory genes that encode for pro-inflammatory molecules, such as TNF-α and IL-6 [14].

In the present study, topical application of IMQ cream on the back of mice caused histological alterations and activated NF-κB molecular pattern that led to the release of pro-inflammatory cytokines and ILs, such as TNF-α and IL-6.

The release of TNF-α in psoriatic lesions activates a positive feedback loop by which the cytokine, in turn, stimulates NF-κB, worsening the inflammatory process [40]. This suggests that NF-κB plays a key role in triggering the inflammatory cascade that characterizes psoriasis. In our study, IMQ animals demonstrated an increased expression of pNF-κB and consequently of TNF-α and IL-6. Treatment with BAY 11-7082, an I-κB kinase-β inhibitor, reduced both pNF-κB and TNF-α, ameliorating inflammatory mediators and preventing pro-inflammatory TNF-α/NF-κB positive feedback activation in psoriatic-like plaques.

STAT3 is activated in inflammatory skin diseases and it has been found, in its phosphorylated form, in psoriatic lesions. Moreover, it has been demonstrated that STAT3 plays a pivotal role in IL-23/Th17 pathway, which is critical for inflammation in the pathogenesis of psoriasis [9]. In the present study, BAY 11-7082 reduced IL-6 expression and likely blunted the activation of the transcriptional factor STAT3 and the message for IL-23, thus ameliorating the inflammatory process.

I-κB phosphorylation activates NF-κB that moves into the nucleus and triggers not only inflammatory genes, but also anti-apoptotic gene expression [41]. Apoptosis regulates proliferation and epidermal growth in normal skin but, by contrast, it has been demonstrated that the apoptotic process is reduced in psoriasis [42], in turn enhancing keratinocytes proliferation. Therefore, we hypothesized that the use of this I-κB kinase-β inhibitor could activate the apoptosis machinery. In fact, our experimental results revealed that apoptosis was reawakened by BAY 11-7082 administration. We observed that Bcl-2, which upon activation down-regulates apoptosis, was significantly expressed in IMQ animals compared with Sham animals; thus reactivating cell proliferation. By contrast, BAY 11-7082 promoted apoptosis as a protective and compensatory mechanism against hyperproliferation, concomitantly stimulating caspase-3 and pJNK, considered pro-apoptotic molecules.

Previous experimental evidence has already demonstrated that NLRP3 inflammasome is activated in inflammatory skin diseases; furthermore, it has been suggested that BAY 11-7082 may be considered an inhibitor of this molecular platform [31,32]. These findings led us to investigate whether NLRP3 inflammasome is involved in IMQ-induced skin lesions and whether the administration of BAY 11-7082 could represent a therapeutic approach as a double inhibitor of both of I-κB and NLRP3. Indeed, our results showed that psoriasis-like lesions were reduced in NLRP3−/− KO mice compared with the skin of wild-type mice challenged with IMQ. More specifically, we observed that the total histological score and epithelial thickness were lower in NLRP3−/− KO mice. Interestingly, psoriasis-like skin of NLRP3−/− KO mice showed persistent expression of pNF-κB and pSTAT3 and both acanthosis and dermal thickness following IMQ were still persistent in NLRP3−/− KO animals, thus confirming that NLRP3 is only partially involved in the development of psoriasis. These findings, coupled with the evidence that BAY 11-7082 reduced the two transcriptional factors and ameliorated skin lesions, also reducing keratinocyte hyperproliferation, led us to hypothesize that the role of NLRP3 might be related to the maintenance of the disease, rather than in its onset. Additionally, a dual inhibition of both pNF-κB and NLRP3 is required to achieve a significant anti-psoriasis effect, at least in the IMQ model. Finally, all these results, taken together, suggest that the mechanism underlying the beneficial effects of BAY 11-7082 in IMQ-induced psoriasis is linked to the blockade of both pNF-κB and NLRP3. Indeed, BAY 11-7082 has demonstrated a broad-spectrum inhibitory activity against inflammatory signalling pathways including phosphoinositide 3-kinase (PI3K)/Akt/IKK/NF-κB, ERK/JNK/AP-1, TBK1/IRF-3 and JAK-2/STAT-1 [43]. Due to this broad anti-inflammatory activity strongly related to Toll-like receptor-activated pathways, it is not possible to rule out that the observed effects on either C57BL/6J and NLRP3−/− KO animals are dependent on other effects of BAY 11-7082.

In conclusion, our data suggest that BAY 11-7082 represents an interesting approach for the management of psoriasis. Its beneficial effect is dependent on an NF-κB and NLRP3 inhibition that interrupt the pathological mechanisms underlying the triggering and the maintenance of psoriasis lesions.

Funding

This work was supported by a liberal donation (to F.S.).

Author contribution

Natasha Irrera and Mario Vaccaro conceived and designed the study; Giovanni Pallio, Gabriele Pizzino, Maria Lentini, Michele Scuruchi and Roberta Ettari obtained the data; Alessandra Bitto, Vincenzo Arcoraci, Letteria Minutoli, Giuseppina Cutroneo and Giuseppe Pio Anastasi analysed and interpreted data. Francesco Squadrito and Domenica Altavilla led the design and drafted the manuscript.

Abbreviations

    Abbreviations
     
  • ABC

    avidin biotin complex

  •  
  • AKT

    protein kinase B

  •  
  • ASC

    apoptosis-associated speck-like protein containing a caspase-recruitment domain

  •  
  • BAX

    Bcl-2-associated X protein

  •  
  • BCL-2

    B-cell lymphoma 2

  •  
  • ERK

    extracellular signal–regulated kinase

  •  
  • H&E

    haematoxylin and eosin

  •  
  • IκB

    inhibitor of κB

  •  
  • IKK

    inhibitor of kappaB kinase

  •  
  • i.p.

    intraperitoneal

  •  
  • IL-1β

    interleukin-1beta

  •  
  • IMQ

    imiquimod

  •  
  • JAK-2

    Janus-Activated Kinase 2

  •  
  • KO

    knockout

  •  
  • NF-κB

    nuclear factor-κB

  •  
  • NLRP3

    NOD-like receptor family

    pyrin domain containing 3

  •  
  • NOD

    nucleotide-binding oligomerization domain

  •  
  • p-JNK

    phospho-c-Jun NH2-terminal Kinase

  •  
  • s.c.

    sub cutaneous

  •  
  • STAT3

    signal transducer and activators of transcription 3

  •  
  • Th

    T helper

  •  
  • TNF-α

    tumour necrosis factor-α

  •  
  • WT

    wild-type

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Author notes

*

These authors contributed equally to this article.